1. Field of the Invention
This invention relates to a liquid level indicator for liquefied gases at extremely low temperatures. More particularly, it relates to a liquid level indicator of the type which exploits the fact that when a superconductor wire is immersed in, e.g., liquid helium, its resistance varies in proportion as the liquid level of the helium rises or falls and in which the terminal voltage of the superconductor wire supplied with a current in advance is measured so as to gauge the liquid level.
2. Description of the Prior Art
Heretofore, liquid level indicators of the described type have been fabricated for laboratory use, and some have been commercialized. These liquid level indicators, however, are intended merely for liquid helium under the normal pressure (at 4.2 .degree. K). Accordingly, when the rate of evaporation of liquid helium has become high under a reduced-pressure state or when the surface of the liquid is in the superfluid state, the resistance of the superconductor wire, which is very greatly dependent upon the temperature, is subject to the influence thereof and a large dispersion arises in measured values. At the time of high pressures, the rate of evaporation of liquid helium becomes low, which is also a problem in degrading the reliability of measured values.